Mechanical properties were determined in living adult rat brain. Reconstructed magnetic resonance images of the rat brain before and after 2 mm compression with a 4.06 mm diameter vinyl screw showed the total volumetric strain was maximal at the site of indentation. The pressure response to stepwise brain compression showed a linear relationship to the point of respiratory compromise. Instrumented indentation was performed on live brain with intact dura using a 4-mm-diameter flat punch indenter to a maximum depth 1.2 mm at loading-unloading rates not exceeding 0.34 N/min and 1.2 mm/min. The calculated elastic modulus showed no consistent change after death. Creep deformation over 15 s was 7.86+/-1.6% in live brain and 27.0+/-0.24% after death. In constant multicycle indentation, displacement from 1st to 10th cycle increased 8.0+/-1.7% in life and 12.9+/-2.8% after death. The results suggest that elastic properties of rat intracranial contents do not change immediately after death, while changes in the viscous properties are substantial. The process of measuring these properties can alter physiological parameters.